DEC 3000/600 Alphaserver problems

[Peter Coghlan]

I have a rackmount DEC 3000/600 Alphaserver containing a H7816-BA power supply
which doesn't work.  As I mentioned before, this power supply is a real pig
to work on.

I also have two tabletop DEC 3000/600 Alphaservers which contain H7816-AA
power supplies which are similar but not identical.  The main difference
seems to be that the H7816-AA power supplies contain four integrated fans
(and lots of dust) while the H7816-BA relies on one large fan external to
it.  I am hoping that comparisons between a working H7816-AA and the failed
H7816-BA will help diagnose the cause of the failure.

So I dug out one of the tabletop machines, took the cover off and powered
it up to ensure the power supply actually works before I go any further.
The green LED came on, the fans turned and the diagnostic LEDs lit up.
After running for less than a minute, there was a cracking/popping noise
and a small spark visible from somewhere around three unmarked orange
surface mount components on the I/O board (not in the power supply) plus
a whiff of cooked electronics.  The power supply kept running and the
diagnostic LEDs remained lit but I powered off quickly and poked around
with my finger to try to find a hot component without success.  I examined
the area with a magnifying glass under a strong light but I was unable to
find anything damaged.

I wonder would these orange components be tantalum supply decoupling
capacitors?  They seem to have a small pip in the solder at the positive
ends.  After powering back on, two of them had 5.0V across them and there
was 4.87V across the third slightly smaller one, there were no further
fireworks (so far anyway) and the SROM mini-console works.

I have had similar experiences with one or both of my two tabletop units
in the past and I was unable to track down the culprits then either.
Neither unit works properly due to cache failure issues.  I wonder could
it be plausable that the caches are not functioning correctly due to
prior failure of several supply decoupling capacitors on the system board?

Unless these devices have markings on the underside, I have no idea what
their capacitance values are.  There is probably no point in trying to
measure them in-circuit if there are several all over the board in parallel
with each other.  Unsoldering them and trying to measure them then isn't
going to work either if they have failed.   Any suggestions for something
to replace them with that is less likely to go bang in the future?  Could
I get away with using components with leads to replace them instead of
surface mounted ones or would that introduce too much inductance?

Regards,
Peter Coghlan.